What Is Phantom Limb Syndrome?

Individuals report feeling their missing limb move, sweat, numb, cramp, and burn

Yuji Karaki/Getty Images.

Phantom limb syndrome is a condition in which individuals experience sensations like pain, touch, and movement in an arm or leg that is no longer attached to the body. About 80 to 100 percent of amputees experience phantom limbs. The sensation can also occur in individuals who have been born without a limb. The time it takes for a phantom limb to appear varies. Some individuals experience the sensation immediately after an amputation, while others do not feel the phantom limb for several weeks.

Despite their name, phantom limb sensations are not restricted to limbs and can occur in many other areas of the body. They have been reported after breast amputations, the removal of parts of the digestive system, and the removal of eyes. 

Types of Sensations in Phantom Limbs

The sensations associated with a phantom limb vary considerably, from a slight tingling feeling to a vivid sensation of a moving limb. Individuals have reported feeling the phantom limb move, sweat, numb, cramp, burn, and/or change in temperature.

While some individuals report that they can voluntarily move the limb – for instance, to shake someone’s hand – others state that the phantom limb stays “habitually” in a certain posture, such as a flexed arm or an extended leg. This habitual position can be very painful, like an arm stretched permanently behind the head, and sometimes replicates the position of the limb before it was amputated.

The phantom limb does not necessarily represent the missing limb accurately. For example, some patients have reported having short arms with missing elbows. Over time, phantom limbs have been observed to “telescope,” or shrink into the stump after amputation. For instance, an arm might shorten progressively until only the hand is attached to the stump.

Such telescoping, which is often associated with increasingly painful phantom limbs, can occur overnight or gradually over years.

The Causes of Phantom Limb Pain 

A number of mechanisms have been proposed as potential factors in phantom limb pain. While none of these mechanisms have been proven to be the root cause of the pain, each theory provides valuable insight into the complex systems at work when a patient experiences a phantom limb sensation.

Peripheral nerves. One formerly dominant mechanism concerning phantom limb pain involves peripheral nerves: nerves that are not in the brain and spinal cord. When a limb is amputated, many severed nerves are left in the amputated stump. The ends of these nerves can grow into thicker nerve tissues called neuromas, which can send abnormal signals to the brain and result in painful phantom limbs. 

However, while neuromas can occur when limbs are amputated, they do not necessarily cause phantom limbs. Phantom limb pain can still occur, for example, in people born without a limb, and so are not expected to have severed nerves from amputation. The limbs can also remain painful even after the neuromas have been removed surgically. Finally, many amputees develop phantom limbs immediately after amputation, before enough time has elapsed for neuromas to develop.

Neuromatrix theory. This theory came from the psychologist Ronald Melzack, who postulated that each person has a network of many interconnected neurons called the neuromatrix. This neuromatrix, which is prewired by genetics but modified by experience, produces characteristic signatures that tell an individual what their body is experiencing and that their body is their own.

However, the neuromatrix theory assumes that the body is intact, with no limbs missing. When a limb is amputated, the neuromatrix no longer receives the input it's accustomed to, and sometimes receives high levels of input because of damaged nerves. These changes in input modify the characteristic signatures produced by the neuromatrix, resulting in phantom limb pain. This theory explains why people born without limbs can still experience phantom limb pain, but is difficult to test.

Furthermore, it is unclear why the neuromatrix would produce pain and not other sensations.

Remapping hypothesis. The neuroscientist Ramachandran proposed the remapping hypothesis to explain how phantom limbs arise. The remapping hypothesis involves neuroplasticity – that the brain can reorganize itself by weakening or strengthening neural connections – occurring in the somatosensory cortex, which is responsible for the body’s sense of touch. Different areas of the somatosensory cortex correspond to different parts of the body, with the right side of the cortex corresponding to the left half of the body and vice versa.

The remapping hypothesis says that when a limb is amputated, the brain area corresponding to that limb no longer receives input from the limb. Neighboring areas of the brain can then “take over” that brain area, causing phantom limb sensations. For example, one study found that people who have had their hand amputated can feel as if their missing hand is touched when a part of their face is touched. This occurs because the brain area corresponding to the face lies beside the brain area corresponding to the missing hand and “invades” the area after amputation.

The remapping hypothesis has gained a great deal of traction in neuroscience research, but it may not explain why patients feel pain in their phantom limbs. In fact, some researchers claim the opposite: rather than having a reduced brain area corresponding to the missing hand because of a brain area taking over, the representation of the hand in the brain was preserved.

Future Research

Although phantom limb syndrome is prevalent among amputees and even occurs in people born without limbs, the condition is highly variable from person to person, researchers have yet agreed on its exact causes.  As research progresses, scientists will be able to better pinpoint the precise mechanisms causing phantom limbs. These discoveries will ultimately lead to the development of better treatments for patients.

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